Communication between cells in the nervous system is dependent on the precise control of neurotransmitter release. The rapid onset and termination of evoked release indicate that the specific proteins that participate in the regulation of transmitter release, it is apparent that the selective targeting of presynaptic proteins to release sites occurs rapidly following specific contact between a nerve cell and its target. Ion channels (especially Ca++ and calcium-activated K+) are known to be of critical importance in the regulation of transmitter release, and are among the proteins known to be selectively targeted to presynaptic active zones. Little is known, however, about the identity, properties, and development of nerve terminal ionic currents. The applicant proposes to study the induction of presynaptic specialization (Ca++ and calcium-activated K+ channels) expressed at transmitter-releasing varicosities that form along neurites in cultures of Xenopus spinal cord neurons and myocytes. These are newly formed synapses where there are no surrounding glial cells to interfere with access. Using patch clamp techniques, the applicant proposes to (1) characterize directly the types of currents present at newly formed presynaptic structures, (2) determine their role in transmitter release regulation, (3) identify cell-cell interactions that regulate the expression of these specializations, and (4) identify basal lamina components that can induce the expression of presynaptic specialization. From these studies should come a more thorough understanding of motor nerve terminal Ca++ and calcium activated K+ channels, their role in transmitter release and the induction of presynaptic specialization.